test: mock repo clone git execution

[euxaristia]

Summary

• route git process execution through a package-level runner hook
• mock git execution in repo clone tests and assert generated clone args
• remove the real SSH/network dependency that caused CI to time out in TestRepoClone_SSH

Verification

• env GOCACHE=/private/tmp/gitee-cli-go-build-cache GOPATH=/private/tmp/gitee-cli-go-path go test -race ./internal/cmd -run 'TestRepoClone_(SSH|WithOptions|InvalidRepo)$'

Note: full local go test ./internal/cmd still fails in this sandbox at TestNewRootCmd_ConfigLoadError, unrelated to the clone timeout.

[gemini-code-assist]

Code Review

This pull request introduces a mockable git command runner to improve the testability of git-related operations. It refactors the existing execution logic to use this runner and adds detailed assertions in the repository clone tests to verify command arguments. Feedback focuses on enhancing the implementation's robustness and testability, specifically by moving the runner into the App struct to support parallel testing, incorporating context.Context for better process management, and using cmd.InOrStdin() for flexible input handling. Additionally, it is recommended to refactor duplicated mocking logic in tests into a shared helper function.